1. The Field of the Invention
The present invention relates to the production of hydrogen gas. More particularly, the present invention provides methods, materials, and apparatus for the production of hydrogen gas from ammonia. In one aspect, the hydrogen released from the ammonia is used as a fuel.
2. The Relevant Art
The efforts made by the U.S. and other industrialized countries at controlling polluting emissions over the past thirty years have greatly reduced the amounts of air-borne pollutants that have been linked with serious environmental and heath risks. In the United States, for example, most urban areas have markedly improved air quality than even a decade ago on average. Nevertheless, much needs to be done to reduce still further the harmful chemicals pumped into the atmosphere from our combustion-driven economy. Indeed, one of the main goals of the automotive industry is the development of low-emission engine technology for vehicles having reduced, or even null, environmental impact. California, for example, will soon require that at least 2% of the vehicles sold in that state produce no polluting emissions.
Meeting these challenges is an extremely difficult proposition. All present vehicles use internal combustion engines (ICEs), in which the explosive reaction of a mixture of a fuel (typically a hydrocarbon (C.sub.n H.sub.2n+2)) and air (principally a mixture of nitrogen and oxygen) is exploited to produce the energy required for vehicle propulsion. However, the combustion of hydrocarbons in air generates various polluting gases, including carbon monoxide (CO), carbon dioxide (CO.sub.2), ozone (O.sub.3), nitrogen and sulfur oxides (i.e., NO.sub.x and SO.sub.x), aldehydes, hydrocarbons, and lead compounds (Greenwood and Earnshaw 1984). CO.sub.2 has achieved notoriety as a "greenhouse gas" for its ability to trap infrared radiation and thus prevent the release of heat from the Earth's atmosphere. Ozone has been linked with respiratory ailments and is a strong oxidant, thus contributing to property damage from air pollutants. The nitrogen and sulfur oxides have been implicated in acid rain, a major environmental concern, as well as property damage resulting from the formation of nitrogen and sulfur acids upon contact with water (H.sub.2 O) in the atmosphere. The nitrogen and sulfur acids are of especial concern to European nations as acid rain has been implicated in the destruction of such well known ancient structures as the Coliseum in Rome and the Parthenon in Athens. The remaining gases have been linked to a variety of health concerns, especially lead, which has been linked to brain damage.
A number of possible replacements to hydrocarbons fuels are under study. Among the various solutions considered, hydrogen (H.sub.2) appears to be a very promising alternative for a number of reasons. Use of hydrogen fuels in ICEs would require only minor modifications to existing engine designs. The combustion of hydrogen in air yields water, which, of course, poses no significant environmental problem, and produces a relatively large amount of energy. In alternative propulsion technologies, such as electric vehicles, hydrogen can be used in fuel cells where it combines with atmospheric oxygen in a more controlled way than combustion through an electrochemical reaction. Electric energy generated by the fuel cell can either be stored in batteries or used directly to feed an electric motor to power the electric vehicle. In this latter case, the efficiency is much higher than in ICEs, approaching values of about 90% compared to values of about 30% which are typical for ICEs. Although fuel cell driven engines still need to be optimized before they can be exploited on an industrial scale, they are expected eventually to gain a significant role in transportation technology. One problem with hydrogen, however, is its safe handing: Hydrogen reacts explosively with air. As a consequence the use of hydrogen as a fuel for wide-spread distribution, in either its gaseous or liquid form (which would also require expensive refrigeration), poses numerous safety, technical, and economic problems that make its use as a fuel prohibitively difficult.
One approach to resolving the drawbacks of hydrogen as a fuel includes considering less expensive, simpler, and cheaper materials that can act as hydrogen carriers. Ammonia (NH.sub.3) has been identified as a suitable hydrogen carrier: Ammonia is essentially non flammable and is readily obtained and handled in liquid form without the need for expensive and complicated refrigeration technology. In addition, ammonia contains about 1.7 times as much hydrogen as liquid hydrogen for a given volume in its liquid form; thus allowing for more efficient transportation of hydrogen fuel. Ammonia can be disproportionated into hydrogen and nitrogen (N.sub.2) in a suitable separation unit upstream of the engine according to the reaction: EQU 2NH.sub.3 .fwdarw.3H.sub.2 +N.sub.2.
The nitrogen can be released to the atmosphere without significant environmental impact. Ammonia can be present in the hydrogen/oxygen fuel mixture in low amounts, up to about 5% by volume of the fuel mixture, without significantly affecting the combustion of hydrogen. In fact, while pure ammonia burns with difficulty in air, it burns easily when mixed to hydrogen. Thus, the dissociation yield of the unit need not be 100%. Furthermore, ammonia has a significant vapor pressure (approximately 100 pounds per square inch (psi) at 27.degree. C.).
The use of ammonia as a storage medium for hydrogen fuel has been disclosed, for example, in U.S. Pat. Nos. 4,478,177 and 4,750,453, both assigned to Valdespino and incorporated herein by reference for all purposes. These patents describe an ICE fueled by hydrogen that is obtained through disproportionation of ammonia in a separation chamber. The ammonia separation unit is a chamber containing a catalyst that is taught to be one or more metals including iron (Fe), nickel (Ni), osmium (Os), zinc (Zn), and uranium (U). These metals are well known materials for dissociating ammonia. Iron-based disproportionation catalysts are described as well (Georgiev 1989). However, low flow rates of ammonia and/or high catalyst temperatures are required for the disproportionation of ammonia over these metals. Another material found useful for cracking ammonia into hydrogen and nitrogen is the alloy comprising, by weight, 40.5% Zr, 24.5% Mn, and 25% by weight Fe (available commercially under the tradename St 909 from SAES Getters of Lainate, Italy) with 10% aluminum (Al) used as a binder (Baker et al. 1994).
Unfortunately, known ammonia cracking technologies are insufficient to enable the use of hydrogen fuels in ICEs. In particular, the above-described low flow rate of present ammonia cracking catalysts prevents the employment of hydrogen as a fuel either for ICEs or fuel cells. A preliminary calculation (Brabbs 1978) has shown that for hydrogen to become a real alternative to hydrocarbons as a fuel for ICEs, a flow of hydrogen of between about 100 standard liters per minute (slm) and about 200 slm to the engine is required.
Thus, it would be of great advantage to identify and develop materials capable of catalyzing the disproportionation of ammonia at rates sufficient for use in ICEs or fuel cells to access the enormous potential of hydrogen fuels as an environmentally sound energy source.